U.S. patent application number 12/869573 was filed with the patent office on 2011-09-29 for systems and methods for invariant pulse latency coding.
Invention is credited to Eugene M. Izhikevich, Csaba Petre, Botond Szatmary.
Application Number | 20110235698 12/869573 |
Document ID | / |
Family ID | 44656463 |
Filed Date | 2011-09-29 |
United States Patent
Application |
20110235698 |
Kind Code |
A1 |
Petre; Csaba ; et
al. |
September 29, 2011 |
SYSTEMS AND METHODS FOR INVARIANT PULSE LATENCY CODING
Abstract
Image processing systems and methods extract information from an
input signal representative of an element of an image and to encode
the information in a pulsed output signal. A plurality of channels
communicates the pulsed output signal, each of the plurality of
channels being characterized by a latency. The information may be
encoded as a pattern of relative pulse latencies observable in
pulses communicated through the plurality of channels and the
pattern of relative pulse latencies is substantially insensitive to
image contrast and/or image luminance. A filter can be employed to
provide a generator signal based on the input signal and pulse
latencies can be determined using a logarithmic function of the
generator signal. The filter may be temporally and/or spatially
balanced and characterized by an integral along spatial and/or
temporal dimensions of the filter that is substantially zero for
all values of a temporal and/or a spatial variable.
Inventors: |
Petre; Csaba; (San Diego,
CA) ; Szatmary; Botond; (San Diego, CA) ;
Izhikevich; Eugene M.; (San Diego, CA) |
Family ID: |
44656463 |
Appl. No.: |
12/869573 |
Filed: |
August 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61318191 |
Mar 26, 2010 |
|
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|
Current U.S.
Class: |
375/240.01 ;
375/E7.026 |
Current CPC
Class: |
H04N 19/107 20141101;
G06K 9/46 20130101; G06T 2207/10016 20130101; G06T 9/002 20130101;
H04N 19/00 20130101; G06T 7/20 20130101; H04B 14/026 20130101; G06N
3/049 20130101; G06T 7/40 20130101 |
Class at
Publication: |
375/240.01 ;
375/E07.026 |
International
Class: |
H04N 11/04 20060101
H04N011/04 |
Claims
1. An image processing system comprising: an input signal
representative of an element of an image; a processor configured to
extract information from the input signal and to encode the
information in a pulsed output signal; and a plurality of channels
that communicate the pulsed output signal, each of the plurality of
channels being characterized by a latency, wherein the information
is encoded as a pattern of relative pulse latencies observable in
pulses communicated through the plurality of channels, and wherein
the pattern of relative pulse latencies is insensitive to one or
more of image contrast and image luminance.
2. The system of claim 1, further comprising a filter that provides
a generator signal based on the input signal, wherein the pulse
latencies are determined using a logarithmic function of the
generator signal.
3. The system of claim 2, wherein the pattern of relative pulse
latencies is insensitive to image contrast and image luminance.
4. The system of claim 2, wherein the pattern of relative pulse
latencies is insensitive to image contrast, and wherein the filter
comprises a linear filter.
5. The system of claim 2, wherein the pattern of relative pulse
latencies is insensitive to image contrast, and wherein the
logarithmic function is applied to a rectified version of the
generator signal.
6. The system of claim 2, wherein the filter is spatially balanced
and characterized by an integral along spatial dimensions of the
filter that is substantially zero for all values of a temporal
variable.
7. The system of claim 2, wherein the filter is temporally balanced
and characterized by an integral along a temporal dimension of the
filter that is substantially zero for all values of a spatial
variable.
8. The system of claim 2, wherein the filter is balanced and
characterized by an integral along all dimensions of the filter
that is substantially zero.
9. The system of claim 1, and wherein the pattern of relative pulse
latencies is insensitive to image contrast, wherein the pulse
latencies each include a common latency associated with the image
contrast.
10. The system of claim 1, wherein the pattern of relative pulse
latencies is insensitive to image luminance, and further comprising
a decoder that decodes the pulsed output signal based on the
relative pulse latencies.
11. The system of claim 10, wherein the decoder receives the pulsed
output signal from the plurality of channels.
12. The system of claim 11, wherein the decoder detects coincident
arrival of pulses on different ones of the plurality of
channels.
13. The system of claim 11, wherein the decoder produces secondary
information and encodes the secondary information into the timing
of pulses in a decoder output.
14. The system of claim 10, wherein the decoder receives the pulsed
output signal from the plurality of channels and at least two of
the plurality of channels have transmission delays that are
different from one another.
15. An image processing system comprising: an input signal
representative of an element of an image; a processor configured to
extract information from the input signal and to encode the
information in a pulsed output signal; a plurality of channels that
communicate the pulsed output signal, wherein the information is
encoded in a pattern of relative pulse latencies between the
channels; and a filter that provides a generator signal based on
the input signal, wherein the pulse latencies are determined using
a logarithmic function of the generator signal.
16. The system of claim 15, wherein the pattern of relative pulse
latencies is insensitive to image luminance, and wherein an offset
and a base of the logarithmic function are optimized to obtain a
most relevant range of the generator signal.
17. The system of claim 16, wherein the range is most relevant when
set to match the dynamic range of the latency values and the
dynamic range of the image signal.
18. The system of claim 15, wherein the pattern of relative pulse
latencies is insensitive to image contrast, wherein pulsed outputs
are generated in the pulsed output signal upon occurrence of one or
more of a cyclic event, arrival of an input frame, appearance of a
new feature in the image and a time related to a previous
event.
19. The system of claim 18, wherein the image processing system is
at least partially embodied in a prosthetic device.
20. The system of claim 18, wherein the image processing system is
at least partially embodied in an autonomous robot.
21. The system of claim 18, wherein the image processing system is
at least partially embodied in a remote server.
22. The system of claim 15, further comprising bounding the
latencies of the pulses within a selected interval.
23. The system of claim 15, wherein the plurality of channels
comprises one or more virtual channels carried in a physical
transmission medium.
24. The system of claim 15, wherein the filter is motion
sensitive.
25. The system of claim 24, wherein the filter is direction
sensitive.
26. A method of processing a signal representative of an image
comprising the steps of: receiving an input signal representative
of an image that includes pulsed signals carried in a plurality of
channels; obtaining a generator signal from the input signal; and
determining relative latencies of two or more pulses in the pulsed
signals using a logarithmic function of the generator signal,
wherein information is encoded in a pattern of the relative
latencies, wherein the pattern of the relative latencies is
insensitive to changes in one or more of image luminance and image
contrast.
27. The method of claim 26, wherein the pattern of the relative
latencies is insensitive to changes in image luminance and image
contrast.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present Application claims priority from U.S.
Provisional Patent Application No. 61/318,191 filed Mar. 26, 2010,
entitled "Systems and Methods For Invariant Pulse Latency Coding,"
which is expressly incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to a computer vision
processing systems and more particularly to systems that encode
visual signals into pulse-code output having information encoded
into pulse timing.
[0004] 2. Description of Related Art
[0005] It is known in the field of neuroscience that neurons
generate action potentials, often called "spikes", "impulses", or
"pulses" and transmit them to other neurons. Such pulses are
discrete temporal events, and there could be many pulses per unit
of time. Conventionally, bursts of a few spikes are considered to
be pulses. It is not known how the brain processes information
based on the timing of pulses or how visual features may be encoded
using pulse-timing.
BRIEF SUMMARY OF THE INVENTION
[0006] Certain embodiments of the present invention comprise
systems and methods for encoding visual signals into pulse-code
output, where the information is transmitted by the relative
timings of pulses. The advantage of the invention is that the
signal-to-pulse encoding is insensitive with respect to the
luminance and the contrast of the input signals.
[0007] The present invention relates generally to a computer vision
system that encodes visual signals into pulse-code output, where
information is encoded into the timing of pulses. It is motivated
by neuroscience findings that timing of pulses is important for
information transmission and processing. This invention is useful
for implementation of the function of an artificial retina in
information processing, robotic, or prosthetic devices.
[0008] In certain embodiments systems and methods are provided that
address issues associated with slow adaptation of pulse-time code
to low or high levels of luminance and contrast. Certain
embodiments provide systems and methods of decoding the pulse-time
code to extract features of the visual signal independently from
their luminance and contrast.
[0009] Certain embodiments of the invention provide an image
processing system that typically comprises a processor configured
to execute instructions maintained in a storage medium, wherein the
instructions cause the processor to process a signal representative
of at least a portion of an image to extract and analyze
information in the signal.
[0010] Certain embodiments of the invention provide image
processing systems and methods that extract information from an
input signal representative of an element of an image signal and to
encode the information in a pulsed output signal. A plurality of
channels communicates the pulsed output signal, each of the
plurality of channels being characterized by a latency. The
plurality of channels may comprise one or more virtual channels
carried in a physical transmission medium. The information may be
encoded as a pattern of relative pulse latencies observable in
pulses communicated through the plurality of channels and the
pattern of relative pulse latencies is substantially insensitive to
image contrast and/or image luminance. In some embodiments, the
pattern of relative pulse latencies is insensitive to both image
contrast and image luminance. Each of the pulse latencies may
include a common latency associated with the image contrast. The
latencies of the pulses may be bounded within a selected
interval.
[0011] Certain embodiments comprise a filter that provides a
generator signal based on the input signal. The pulse latencies can
be determined using a logarithmic function of the generator signal.
The logarithmic function may be applied to a rectified version of
the generator signal. The filter may comprise a linear filter. The
filter may be spatially balanced and characterized by an integral
along spatial dimensions of the filter that is substantially zero
for all values of a temporal variable. The filter may be temporally
balanced and characterized by an integral along a temporal
dimension of the filter that is substantially zero for all values
of a spatial variable. In some embodiments, the filter can be
balanced and characterized by an integral along all dimensions of
the filter that is substantially zero.
[0012] Certain embodiments comprise a decoder that decodes the
pulsed output signal based on the relative pulse latencies. The
decoder may receive the pulsed output signal from the plurality of
channels and may be configured to detect coincident arrival of
pulses on different ones of the plurality of channels. Typically,
at least two of the plurality of channels have transmission delays
that are different from one another. The decoder may produce
secondary information and may encode secondary information into the
timing of pulses in an output of the decoder.
[0013] Certain embodiments of the invention provide an image
processing system that comprises a processor configured to extract
information from a signal representative of an element of an image
and to encode the information in a pulsed output signal. A
plurality of channels communicates the pulsed output signal. The
information can be encoded in a pattern of relative pulse latencies
between the channels. The pattern of relative pulse latencies can
be insensitive to image luminance and/or image contrast. A filter
can be configured to provide a generator signal based on the input
signal, and a logarithmic function of the generator signal can be
used to determine the latencies of the pulses. An offset and a base
of the logarithmic function can be configured and optimized to
obtain a most relevant range of the generator signal. The range can
be determined to be most relevant when set to match the dynamic
range of the latency values and the dynamic range of the image
signal.
[0014] In certain embodiments, the pattern of relative pulse
latencies is insensitive to image contrast, and pulsed outputs are
generated in the pulsed output signal upon occurrence of one or
more of a cyclic event, arrival of an input frame, appearance of a
new feature in the image and a time related to a previous event.
The image processing system may be embodied, at least in part, in a
prosthetic device, an autonomous robot or in some other
electromechanical device. Portions of the image processing system
can be embodied in a remote server.
[0015] Certain embodiments execute instructions that cause a
processor to perform a method for processing a signal
representative of an image. The method may comprise receiving an
input signal representative of an image that includes pulsed
signals carried in a plurality of channels. The method may further
comprise the step of obtaining a generator signal from the input
signal. The method may further comprise determining relative
latencies of two or more pulses in the pulsed signals using a
logarithmic function of the generator signal. Information can be
encoded in a pattern of the relative latencies. Typically, the
pattern of the relative latencies is substantially insensitive to
changes in image luminance, image contrast or image luminance and
image contrast.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 illustrates the encoding of an input signal into
latency of pulses along three communication channels.
[0017] FIG. 2 illustrates the decoding mechanism of latency code
employed in certain embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Embodiments of the present invention will now be described
in detail with reference to the drawings, which are provided as
illustrative examples so as to enable those skilled in the art to
practice the invention. Notably, the figures and examples below are
not meant to limit the scope of the present invention to a single
embodiment, but other embodiments are possible by way of
interchange of some or all of the described or illustrated
elements. Wherever convenient, the same reference numbers will be
used throughout the drawings to refer to same or like parts. Where
certain elements of these embodiments can be partially or fully
implemented using known components, only those portions of such
known components that are necessary for an understanding of the
present invention will be described, and detailed descriptions of
other portions of such known components will be omitted so as not
to obscure the invention. In the present specification, an
embodiment showing a singular component should not be considered
limiting; rather, the invention is intended to encompass other
embodiments including a plurality of the same component, and
vice-versa, unless explicitly stated otherwise herein. Moreover,
applicants do not intend for any term in the specification or
claims to be ascribed an uncommon or special meaning unless
explicitly set forth as such. Further, the present invention
encompasses present and future known equivalents to the components
referred to herein by way of illustration.
[0019] Although certain aspects of the invention can best be
understood in the context of conversion of visual input to pulse
latency output in retina transmitted through multiple channels
corresponding to retinal ganglion cells, disclosed systems and
methods can be embodied in spatiotemporal filters implementing
visual processing in general. For example, systems and methods
according to certain aspects of the invention can be applied in a
model of animal visual system as well as in the thalamus or cortex
of an animal. Embodiments of the presently disclosed invention may
be deployed in a hardware and/or software implementation of a
computer-vision system, provided in one or more of a prosthetic
device, robotic device and any other specialized visual system. For
example, an image processing system according to certain aspects of
the invention may comprise a processor embodied in an application
specific integrated circuit ("ASIC") that can be adapted or
configured for use in an embedded application such as a prosthetic
device. Certain of the disclosed systems and methods may be used
for processing of signals of other, often non-visual modalities.
Certain of the disclosed systems and methods may be used for
processing signals without spatial or temporal filtering.
[0020] For the purposes of this description, pulses are understood
to refer to any of a single spike, a burst of spikes, an electronic
pulse, a pulse in voltage, a pulse in electrical current, a
software representation of a spike and/or burst of spikes and any
other pulse or pulse type associated with a pulsed transmission
system or mechanism. For the purposes of this description,
insensitivity of signal-to-pulse encoding with respect to luminance
and/or contrast of an input signals may be understood as encoding
that is invariant or substantially invariant to changes in
luminance and/or contrast.
[0021] Certain embodiments of the invention can be used to encode
visual features including features included in an observed scene,
regardless of luminance and contrast. In some embodiments,
information processing relies on different timing aspects of pulse
timing in order to encode sensory input as a pulse-coded output,
which can be used for further information transmission and
processing.
[0022] For convenience and for the sake of illustration, we assume
that the input signal is given as a function I(x,t) of space x and
time t. For example, the function may describe a movie with frame
number t and a two-dimensional image parameterized by the spatial
two-dimensional vector-variable x, as illustrated in FIG. 1 (frames
100-104). One goal is to convert the input signal to a pulse code
over many channels that is invariant to contrasts.
Contrast-Invariant Encoding
[0023] Without loss of generality, the signal may be represented in
the equivalent form:
I(x,t)=L(1-MS(x,t))
where the parameters L and M denote the luminance and the contrast,
and the "feature" S(x,t) has zero mean calculated over space and/or
time.
[0024] Such an image can be analyzed by a plurality of channels,
each having a linear spatiotemporal filter with kernel F(x,s)
satisfying the following "balance" condition:
.intg..intg.F(x,s)dx ds=0 (1)
Each such filter can be applied to the input signal l(x,t) to
obtain a "generator signal" of the corresponding channel
g(t)=.intg..intg.I(x,t-s)F(x,s)dx ds
The generator signal can be used to calculate the timing of pulsed
response relative to the time t, i.e., the latency of response
transmitted over each channel:
Lat=C-log.sub.B|g(t)|.sub.30
where |g(t)|.sub.+ is the rectified value of g(t), i.e., zero for
negative g(t) and equal to g(t) when g(t).gtoreq.0. Other functions
may be used in addition or in place of the piece-wise linear
rectifier | |.sub.+. For the purposes of this description,
"rectifier" can mean a piece-wise linear or other function that is
positive such that the log function is well defined. Parameter C is
the offset and parameter B is the base of the logarithm. These
parameters are typically selected to optimize the efficiency of the
logarithmic conversion, so that the relevant range of the generator
signal g(t) is captured by the desired range of the latencies. For
example, if the generator signal has a range of interest,
[g.sub.min, g.sub.max], and the desirable latency interval is
[l.sub.min, l.sub.max], then C and B can be found from the system
of equations l.sub.min=C-log.sub.B g.sub.max, l.sub.max=C-log.sub.B
g.sub.min. When g(t)=0 or g(t)<0, the latency of pulse may be
assumed to be infinite. Such latency can be interpreted, e.g., as
representative of non-generation of a pulse by a
channel--corresponding to a pulse with infinite latency--or
representative of a pulse generated with a relatively large
latency. When g(t)>g.sub.max, the channel may generate a pulse
with very short latency. Negative latencies may be avoided by a
cutoff at a value of l.sub.min.
[0025] An example of signal to pulse latency encoding is
illustrated in FIG. 1. In the example, the signal is depicted as a
sequence of frames (frames 100-104). The conversion from signals to
pulses occurs at time moments marked by arrows (105-107), which
could occur every frame or at some frames, as shown in the drawing.
Three output channels (111-113) generate pulses (121-126). Each
such channel may have its own spatiotemporal filter, its own
generator signal, and hence its own timing of pulses relative to
the corresponding moment (vertical dashed lines next to arrows
105-107). For the purposes of this description, a latency of a
pulse (e.g., 130 is the latency of the pulse 123) is distinguished
from the difference between latencies (e.g., 131 is the difference
between latencies of pulses 124 and 125), which is referred to
herein as "relative latencies".
[0026] This approach offers the advantage that it results in
contrast-invariant latency code; that is, individual latencies of
pulses may depend on the contrast of the input signal, but relative
latencies do not. Indeed,
g(t)=.intg..intg.I(x,t-s)F(x,s)dx ds
=.intg..intg.L(1+MS(x,t-s))F(x,s)dx ds
=LM .intg..intg.S(x,t-s)F(x,s)dx ds
because of (Eq.1). For the sake of simplicity of notation, it can
be assumed that the generator signal is positive, and | |.sub.+ can
be omitted from the equation. The latency of each channel is
Lat=C-log.sub.B g(t)=C-log.sub.B LM .intg..intg.S(x,t-s)F(x,s)dx ds
=C-log.sub.B LM-log.sub.B .intg..intg.S(x,t-s)F(x,s)dx ds
Thus latency of pulsed response on each channel is shifted by the
constant log.sub.B LM that depends on the luminance and the
contrast of the input signal. However, latencies of all channels
are shifted by the same constant, so the differences between
latencies (relative latencies) are independent of the values L and
M; in particular, they are contrast-invariant.
[0027] The condition (Eq.1) may be referred to as the "balance
condition," which can be satisfied when
.intg.f(x,s) dx=0 (for all s; "spatial balance")
or
.intg.f(x,s) ds=0 (for all x; "temporal balance")
That is, the kernel, F, is zero along the spatial (dx) dimensions
or temporal (ds) dimension, leading to "spatial" or "temporal"
balance. It can also be zero even if neither of the two conditions
above is satisfied, but the integral is evaluated along all
dimensions. In practice, it is typically unnecessary to require
that the integral be exactly zero and a small non-zero number may
be permitted. In this case, the contrast-invariant pulse encoding
will be approximate, i.e., it will contain a small error which is
proportional to the absolute value of the integral in (Eq.1). Since
exact zeros may be difficult to achieve in practice, "approximate
zero" condition may be considered to be a balance condition.
[0028] In certain embodiments, filters other than linear
spatiotemporal filters may be used. The "balance condition" or
"approximate zero" condition may be satisfied for motion sensitive
filters, direction sensitive filters, certain nonlinear filters and
other filters. A motion sensitive filter can comprise any suitable
spatiotemporal filter that is responsive to the movement of a
visual stimulus over the visual field in time. A direction
sensitive filter can comprise a motion sensitive filter that is
more responsive to motion of a stimulus over the visual field in
some subset of all possible directions.
Latency Adaptation
[0029] It can be advantageous to adapt the sensitivity of the
encoding mechanism such that latency within desired bounds
adequately encodes inputs having luminance or contrasts that may
vary over space and/or time by orders of magnitude. In certain
embodiments, the generator signal may be mapped to latencies via
the equation
Lat=C-log.sub.B |g(t)/a(t)|.sub.+
where the "adaptation" variable a=a(t) evolves according to the
differential equation
da/dt=(|g(t)|-a)/.tau.
(or its integral or discrete analogue) where da/dt is the
derivative with respect to time t, |g(t)| is the absolute value of
g(t), and .tau. is the adaptation time constant. The adaptation
variable a(t) keeps track of the "average" value of |g(t)|, so that
the latency indicates deviations from the average value. In another
embodiment, the differential equation for the adaptation variable
may be
da/dt=(-1+(e+|g(t)|)/a)/.tau.
where e>0 is some small number that is used to cap the unbounded
growth of a if g(t)=0 for a long time. It will be appreciated that
a difference between the two equations above is that the generator
signal affects the time constant of adaptation in the latter case,
but not in the former case. In certain embodiments, the equation
may also be
da/dt=(f(g(t))-a)/.tau.
with some function f. A nonlinear (in a) version
da/dt=f(g(t), a)
(or its integral or discrete analogue) is also possible. In this
case, the variable a(t) may reflect the history of g(t) over a
certain time window (possibly infinite, as in low-pass
filtering).
[0030] This mechanism achieves the following desirable functions:
[0031] If g(t) varies between different output values due to
contrast value changes in its input, a(t) will approach the average
of such values of g(t). [0032] If g(t) becomes very small, a(t)
will decrease proportionally so that the ratio g(t)/a(t) approaches
1. [0033] Similarly, if g(t) becomes very large, a(t) will grow and
the ratio g(t)/a(t) will approach 1 too. Thus, the adaptation
variable shifts the latency of pulses so that they always vary
around certain "optimal" values, resulting in temporal contrast
adaptation. In the example above, the optimal latency value, L, is
C-log.sub.B 1=C.
[0034] The adaptation parameter can also be a vector. For example,
the filter F(x,t) may be decomposed into a number of separate
filters that are used to compute separate generator signals, which
are combined to determine the main generator signal. In the visual
system, for example, the filter F(x,t) may have separable center
and surround regions, and hence the adaptation parameter could have
2 values, one for the center and one for the surround. Both, the
center and the surround, can adapt independently, and the
adaptation vector would scale each corresponding generator signal,
thereby affecting the main generator signal.
[0035] An alternative adaptation mechanism may adjust each latency
by a subtractive parameter, i.e.,
Latency=Lat-b(t)
where b(t) depends on the history of the latencies Lat, which are
computed as above. For example, it can be a low-pass filter
db/dt=(P(Lat)-b)/.tau..sub.s
(or its integral or discrete analogue), where P(Lat) is a function
that restricts the values of Lat to a certain interval, e.g., by
ignoring the values where Lat is infinity (which would correspond
to g(t) being negative or zero) and replacing them with a finite
number. Parameter .tau..sub.s is the time constant of the low-pass
filter. One implementation of the low-pass filter functionality is
the running average of P(Lat) over a certain time window. A
nonlinear (in b) version of the equation above
db/dt=f(Lat, b)
is also possible.
[0036] The choice of the nonlinear function f may be different for
different variables (a or b) and for different applications. For
example, the function may make the parameters adapt to the changing
ranges of the magnitude of the input signal, its contrast, or its
spatial and/or temporal statistics.
Input Signal Adaptation
[0037] In addition to the adaptation of the latencies conducted by
the "adaptation variable" a(t) or b(t) and affecting directly the
logarithmic conversion of the generator signal to latencies, it may
be necessary and/or desirable to have an adaptation of the input
signal itself. Such input signal adaptation may be referred to as
"cone adaptation" as if the input signal were the signal coming
from cone photoreceptors of retina, though the method would work
for any other type of signal.
[0038] It is often desirable to take a raw signal I(x,t) and
convert it to a rescaled signal J(x,t) where the values of J(x,t)
at any spatial location, x, are deviations (positive or negative)
from a certain "mid-point" value, which e.g. could be the mean of
I(x,t) at the same location (and hence it would depend on x), or
the mean over the entire signal, or the mean over a part of it.
This way, the rescaled signal J(x,t) reports changes from the mean.
However, if the mean of I(x,t) changes, e.g., due to changed
luminance or contrast, it may be desirable that the rescaling and
conversion to J(x,t) should also change adaptively, thereby
modeling the cones of a retina.
[0039] In certain embodiments. It may be desired that the rescaled
signal has approximately zero mean and deviations of the order of k
from the mean for some constant k that might depend on the
particular software and hardware restrictions. For example, in one
example implementation, a value of k=127 is used when the pixel
values are within the range [0, 255]. This can be achieved if
J(x,t)=I(x,t)p-k
with an appropriate (possibly x-dependent) parameter p that adapts
to the changing statistics of I(x,t) as to keep Ip.apprxeq.k, which
can be achieved through the following equation:
dp/dt=(1-p I(x,t)/k)/.tau..sub.p
Here, .tau..sub.p is the input signal adaptation time constant.
However, the input signal may be absent (i.e., I(x,t)=0) for a long
period of time and, in this case, p will be growing unboundedly
with the grown rate 1/.tau..sub.p. To cope with such condition, an
upper bound may be set for the value of p. A slightly modified
equation may be used:
dp/dt=(1-p [e+I(x,t)]/k)/.tau..sub.p
where e>0 is a small parameter that would play the bounding role
when I(x,t)=0 because p will asymptote at k/e in this case. In one
example, for an input signal encoded as an RGB image with discrete
values between 0 and 255, values of k=127 and e=1 may be used. In
one embodiment, a 1 can be added to all pixels of the input signal
and used the equation with no e.
[0040] In certain embodiments, the equation for signal adaptation
may be
dp/dt=(k/[e+I(x,t)]-p)/.tau.
Notice that the difference between the two equations above is that
the input signal affects the time constant of adaptation in the
former case, but does not in the latter case. A nonlinear version
of the input signal adaptation is also feasible
dp/dt=f(I(x,t), p) (2)
with some function f such that it promotes Ip.apprxeq.k.
[0041] In another embodiment, the rescaled (adjusted) image may be
given by
J(x,t)=I(x,t)-p
where the offset p adapts to the input signal, e.g., via the
low-pass filter differential equation
dp/dt=(I(x,t)-p)/.tau.
or via a nonlinear function (Eq.2).
Decoding
[0042] Certain embodiments have pulsed outputs whose relative
latencies are invariant with respect to signal contrast is
desirable for a decoder, whose job, e.g., may be to perform pattern
recognition of the signal that is independent of attributes such as
contrast. In one example, the decoder comprises a coincidence
detector that signals coincident arrival of pulses. In another
example, the decoder may receive input from the encoder with
different transmission delays, as illustrated in FIG. 2. Such a
decoder may generate a pulsed output when the relative latencies of
pulses match the difference of transmission delays, and ignore
other inputs. The response of such a decoder will typically be
invariant to the contrast of the input signal. In another example,
the decoder may be part of an organic nervous system, such as the
nervous system of an animal which can receive input from a
prosthetic device.
[0043] For example, suppose a signal (201) provided in one image
frame results in two output pulses (221 and 222) generated by two
channels (211 and 212) with certain latency from the time marked by
the dashed line (250). The pulses arrive to the decoder (240) with
certain transmission delays indicated by the arrows (231 and 232).
Because the pulses arrive at different time, the decoder, being a
coincident detector, will not register a coincidence. Now, another
input signal (202) results in pulsed output with latencies (223 and
224) that have relative latencies (i.e., the difference of
latencies) matching the difference of transmission delays. Such
pulses arrive to the decoder at the same time (241) resulting in an
output pulse. Increasing the contrast of the input signal (203)
results in pulsed output (225 and 226) with shorter latencies, yet
the same relative latencies (latency differences), which again
matches the difference between transmission delays.
Thus, signals with different levels of contrast result in pulsed
outputs with different latencies but with the same relative
latencies and they can be readout by a decoder that receives these
pulses along channels with different transmission delays that match
the latency differences. Notice also that the decoder generates a
pulsed output whose latency depends on the latency of the incoming
pulses. Indeed, the latency 261 of the output pulse 241 is longer
than the latency 262 of the output pulse 242.
Additional Descriptions of Certain Aspects of the Invention
[0044] The foregoing descriptions of the invention are intended to
be illustrative and not limiting. For example, those skilled in the
art will appreciate that the invention can be practiced with
various combinations of the functionalities and capabilities
described above, and can include fewer or additional components
than described above. Certain additional aspects and features of
the invention are further set forth below, and can be obtained
using the functionalities and components described in more detail
above, as will be appreciated by those skilled in the art after
being taught by the present disclosure.
[0045] Certain embodiments of the invention provide an image
processing system. Some of these embodiments comprise an input
signal representative of an element of an image. Some of these
embodiments comprise a processor configured to extract information
from the input signal and to encode the information in a pulsed
output signal. Some of these embodiments comprise a plurality of
channels that communicate the pulsed output signal, each of the
plurality of channels being characterized by a latency. In some of
these embodiments, the information is encoded as a pattern of
relative pulse latencies observable in pulses communicated through
the plurality of channels. In some of these embodiments, the
pattern of relative pulse latencies is insensitive to one or more
of image contrast and image luminance.
[0046] Some of these embodiments comprise a filter that provides a
generator signal based on the input signal. In some of these
embodiments, the pulse latencies are determined using a logarithmic
function of the generator signal. In some of these embodiments, the
pattern of relative pulse latencies is insensitive to image
contrast and image luminance. In some of these embodiments, the
pattern of relative pulse latencies is insensitive to image
contrast. In some of these embodiments, the filter comprises a
linear filter. In some of these embodiments, the pattern of
relative pulse latencies is insensitive to image contrast. In some
of these embodiments, the logarithmic function is applied to a
rectified version of the generator signal. In some of these
embodiments, the filter is spatially balanced and characterized by
an integral along spatial dimensions of the filter that is
substantially zero for all values of a temporal variable. In some
of these embodiments, the filter is temporally balanced and
characterized by an integral along a temporal dimension of the
filter that is substantially zero for all values of a spatial
variable. In some of these embodiments, the filter is balanced and
characterized by an integral along all dimensions of the filter
that is substantially zero.
[0047] In some of these embodiments, the pattern of relative pulse
latencies is insensitive to image contrast, wherein the pulse
latencies each include a common latency associated with the image
contrast. In some of these embodiments, the pattern of relative
pulse latencies is insensitive to image luminance, and further
comprising a decoder that decodes the pulsed output signal based on
the relative pulse latencies. In some of these embodiments, the
decoder receives the pulsed output signal from the plurality of
channels. In some of these embodiments, the decoder detects
coincident arrival of pulses on different ones of the plurality of
channels. In some of these embodiments, the decoder produces
secondary information and encodes the secondary information into
the timing of pulses in a decoder output. In some of these
embodiments, the decoder receives the pulsed output signal from the
plurality of channels and at least two of the plurality of channels
have transmission delays that are different from one another.
[0048] Certain embodiments of the invention provide an image
processing system. Some of these embodiments comprise an input
signal representative of an element of an image. Some of these
embodiments comprise a processor configured to extract information
from the input signal and to encode the information in a pulsed
output signal. Some of these embodiments comprise a plurality of
channels that communicate the pulsed output signal. In some of
these embodiments, the information is encoded in a pattern of
relative pulse latencies between the channels. Some of these
embodiments comprise a filter that provides a generator signal
based on the input signal, wherein the pulse latencies are
determined using a logarithmic function of the generator
signal.
[0049] In some of these embodiments, the pattern of relative pulse
latencies is insensitive to image luminance. In some of these
embodiments, an offset and a base of the logarithmic function are
optimized to obtain a most relevant range of the generator signal.
In some of these embodiments, the range is most relevant when set
to match the dynamic range of the latency values and the dynamic
range of the image signal. In some of these embodiments, the
pattern of relative pulse latencies is insensitive to image
contrast, wherein pulsed outputs are generated in the pulsed output
signal upon occurrence of one or more of a cyclic event, arrival of
an input frame, appearance of a new feature in the image and a time
related to a previous event. In some of these embodiments, the
image processing system is at least partially embodied in a
prosthetic device. In some of these embodiments, the image
processing system is at least partially embodied in an autonomous
robot. In some of these embodiments, the image processing system is
at least partially embodied in a remote server. Some of these
embodiments comprise bounding the latencies of the pulses within a
selected interval. In some of these embodiments, the plurality of
channels comprises one or more virtual channels carried in a
physical transmission medium.
[0050] Certain embodiments of the invention provide methods for
processing a signal representative of an image. Some of these
methods comprise the step of receiving an input signal
representative of an image that includes pulsed signals carried in
a plurality of channels. Some of these methods comprise the step of
obtaining a generator signal from the input signal. Some of these
methods comprise the step of determining relative latencies of two
or more pulses in the pulsed signals using a logarithmic function
of the generator signal, wherein information is encoded in a
pattern of the relative latencies. In some of these embodiments,
the pattern of the relative latencies is insensitive to changes in
one or more of image luminance and image contrast. In some of these
embodiments, the pattern of the relative latencies is insensitive
to changes in image luminance and image contrast.
[0051] Although the present invention has been described with
reference to specific exemplary embodiments, it will be evident to
one of ordinary skill in the art that various modifications and
changes may be made to these embodiments without departing from the
broader spirit and scope of the invention. Accordingly, the
specification and drawings are to be regarded in an illustrative
rather than a restrictive sense.
* * * * *